It is standard practice to use large metal storage tanks for storing a variety of liquids, such as beverage and petroleum products. Conventional large storage tanks are usually made from non-stainless steel plates, and in the case of petroleum products the storage tanks are generally made from ¼ inch (0.63 cm) to ½ inch (1.27 cm) thick steel plates welded together. The dimensions of conventional large storage tanks usually range in the hundreds of feet (100 feet=30.5 meters) in height and hundreds of feet in diameter.
Non-contact electromagnetic detection and sensing may be used to determine the presence or signatures (object classification or shape) of objects, or levels or the distance to the surface of product materials, when other sensing methods have difficulty in providing reliable and/or accurate information. For example, in the oil and gas industry, inaccurate or unreliable tank level measurements can incur a significant loss of profitability/revenue in tank level gauging applications. An error of 1 millimeter (mm) of the level measurement in bulk storage tanks (40-80 meters in diameter) can correspond to volumetric error of several cubic meters. Since the crude oil price is generally at least $100 per barrel (1 barrel=42 US gallons; 159 liters), the 1-mm error can result in thousands of dollars loss for one or more parties involved in trading and oil transfer.
RAdio Detection And Ranging (Radar) has been used as a type of non-contact product level gauge for several decades. The radar system includes a transmitter coupled to a radar antenna which is positioned above the product (e.g., a liquid or solid) for emitting radar signals to the product and a receiver coupled to the antenna (or to another antenna) for receiving radar signals reflected from the product surface, as well as a signal processor for determining the product level on the basis of the emitted radar signals and the reflected radar signals. According to this method, the antenna driven by transmit circuitry emits a radar signal which strikes an object or surface, for example a liquid surface. The object or surface reflects part of the emitted radar signal/wave back in the direction of the antenna, which is received and is coupled to receive circuitry including a processor that processes the reflected radar signal/wave.
However, radar measurements can be affected by multiple reflections inside storage tanks such as due to tank walls, the tank bottom, the tank roof and tank obstructions including agitators, ladders and the heat coil. Furthermore, every tank generally has to use the maximum capacity for oil storage and transfer. It is thus important that the measurements be constantly reliable as the level of product approaches the bottom or the roof of the storage tank.
In addition, European Union' (EU) regulations have already imposed restrictions on using high radar power in open air and plastic storage tanks due to spectrum interference to other devices and/or systems, and the U.S. Federal Communication Commission (FCC) may devise a new rule similar to EU's. Low transmit power is known to worsen the performance of pulsed radar. Some proposed solutions' include so-called adaptive power control (APC) that reduces the transmit power when there are other devices in the vicinity of the installed radar level gauge system. But simply using reduced power means the measuring distance and/or the signal to noise ratio (SNR) is reduced. As a result, the measurement accuracy is degraded, hence measurement uncertainty and errors are increased.
To improve measurement accuracy, ultra-short radar pulses in the sub-nanosecond range have been proposed. However, the shorter a pulse is, the lower its power, and hence the shorter the measuring distance. Given a specified SNR and measurement accuracy, the maximum measuring distance has to be reduced on one hand. On the other hand, the measurement accuracy is degraded at a longer distance, i.e., reduced reliability of pulse radar system with distance and transparency of the product (e.g., dielectric constant<1.4) to radar waves.